The invention relates to a process for preparing carbonyl compounds from olefins and an oxidizing agent in which the olefin is oxidized in a reactor, a reaction gas mixture which comprises unoxidized olefin being formed.
The invention relates in particular to a process for preparing acetaldehyde by the Wacker-Hoechst process (EP 0 006 523 A1, incorporated into the present application by reference) from the raw materials ethylene and oxygen in the presence of an aqueous catalyst solution (copper(11) chloride and palladium chloride).
The raw materials ethylene and oxygen react, on an industrial scale, in a vertically upright reactor which is filled with the catalyst solution. The ethylene is fed to the reaction by a continuously recirculated gas stream (recycle gas). The gas stream leaving the reactor, which gas stream, in addition to the reaction product acetaldehyde, comprises stream, unreacted ethylene, carbon dioxide (CO2), oxygen (O2) and small amounts of minor components (reaction gas mixture), is conducted off from the reaction compartment and treated by a multistage process.
Water and aldehyde are separated off by a 2-stage condensation (water separation and gas cooler) with subsequent scrubbing. The aldehyde-free recycle gas which, in the standard operating state, can comprise 60-80% by volume ethylene, 3-7% by volume oxygen and 8-25% by volume CO2 as main components, is compressed to compensate for the loss of pressure, admixed with fresh ethylene and fed to the reactor.
To keep the concentrations of ethylene and inert constituents such as CO2, nitrogen, argon, methane and ethane constant in the recyle gas, it is expedient to bleed off a certain gas stream constantly from the system. Because of its considerable ethylene content, it is advantageous to make further use of the gas stream as raw material (world market price of ethylene currently approximately 800-DM/metric ton). For this purpose, it is first cooled to reduce the moisture content, then compressed using compressors and after passing through an absorption dryer, is transmitted via a pipeline to a consumer. If this pathway of offgas utilization is impossible, the offgas must be flared off via a high flare.
Alternatively, there are essentially two possibilities for utilization;
a) thermal utilization and
b) treatment by absorption or adsorption, for example of CO2 in sodium hydroxide solution, and recycling the ethylene to the process.
Ethene regeneration integrated into the process in initially to be preferred to thermal utilization of the offgas. CO2 and O2 could in principle be separated off by an absorption process, but this would be highly complex because of the high CO2 and O2 contents and would intervene in the licensed material cycle.
The object underlying the invention therefore was to improve the process mentioned at the outset by a suitable separation process.
It has now surprisingly been found that this object can be achieved by separating off the unoxidized olefin, ethylene in the specific case, from the reaction gas mixture by the membrane separation process of gas permeation.
Whereas gas permeation in the case of separation of N2/O2, for example, belongs to the prior art, its potential use in the present case is surprising in that the selectivity of commercially available membranes for the component ethylene (C2H4) is generally classified as insufficient for a quantitative separation with acceptable yields. Ethylene has a great similarity to methane (CH4), which is enriched by gas permeation industrially (e.g. in natural gas or landfill gas) only to 60% by volume methane, because the loss of methane is excessive at higher concentrations.
The invention therefore relates to a process for preparing carbonyl compounds from olefins and an oxidizing agent in which the olefin is oxidized in a reactor, a reaction gas mixture which comprises unoxidized olefin being formed, which comprises separating off unoxidized olefin at least in part from the reaction gas mixture by a membrane process.
The invention therefore likewise relates to a process for separating off a gaseous olefin from a gas mixture comprising more than 8% by volume carbon dioxide or oxygen, which comprises separating off the olefin from the gas mixture by gas permeation.
In a first particular embodiment, the carbonyl compound is acetaldhyde and/or the olefin is ethene and/or the oxidizing agent is oxygen. The unoxidized olefin can here either diffuse through the membrane or be retained by it. Possible membranes are polymer membranes, preferably polyimide membranes. However, suitable membranes are also membranes or inorganic materials, preferably of ceramic or metal, palladium or platinum being particularly preferred. In further particular embodiments, the absolute pressure on the reaction gas mixture side is in the range from 1 to 80 bar abs., preferably in the range from 3 to 40 bar abs., or the absolute pressure on the permeate side is less than or equal to 2 bar abs., preferably less than or equal to 1 bar abs., particularly preferably less than or equal to 200 mbar abs. The membrane process is preferably carried out at temperatures in the range from 0 to 100xc2x0 C., preferably in the range from 10 to 40xc2x0 C. The membranes are preferably present in the form of spirally wound modules or hollow fiber modules.
Particular embodiments are given by the features of the subclaims. One or more of these features can also represent, together or each alone, solutions according to the invention of the object and these features can also be combined in any manner.